1. Field of the Invention
The present invention relates to an induction heating apparatus, and more particularly, it relates to an induction heating apparatus which controls the heating output in operation by simultaneously controlling an oscillation frequency and a duty ratio of a high frequency generation circuit.
2. Description of the Prior Art
In general, an induction heating apparatus comprises a high frequency generation circuit which generates an AC current from a DC power source to supply the generated AC current to an induction heating coil for generating an alternating field, thereby to induce and heat a heating apparatus of ferrous metal such as a cooking apparatus which is provided in close vicinity of the induction heating coil.
Generally in such an induction heating apparatus, the heating output supplied to a heating apparatus such as a cooking apparatus has been adjusted by controlling the oscillation frequency and the duty ratio of a high frequency generation circuit as disclosed in Japanese Patent Publication gazette No. 15955/1980.
In the aforementioned conventional induction heating apparatus, the heating output is adjusted by either the oscillation frequency or the duty ratio of the high frequency generation circuit whereby the range of adjustment is restricted, such that the heating output is adjusted by duty ratio control of the high frequency generation circuit when a set output value is lower than a predetermined level while the same is adjusted by oscillation frequency control of the high frequency generation circuit when the set output value exceeds the predetermined level. In the conventional induction heating apparatus, therefore, oscillation frequency control and duty-cycle controlling of the high frequency generation circuit must be appropriately combined in correspondence to the level of the set output value for obtaining a wide range of adjustment of the heating output, leading to complication in structure and operation processes.
For readily adjusting the heating output of the induction heating apparatus over a wide range, the oscillation frequency and the duty ratio of the high frequency generation circuit may be simultaneously controlled.
FIG. 1 is an illustration showing a relation between the volume of adjustment S of an output setting knob and the oscillation period T of a high frequency generation circuit in a conventional induction heating apparatus and FIG. 2 is an illustration showing a relation between the volume of adjustment S of the output setting knob and the duty ratio D of the high frequency generation circuit, while FIG. 3 is an illustration showing a relation between the volume of adjustment S of the output setting knob and the heating output P upon simultaneous performance of oscillation frequency control and duty-cycle controlling of the high frequency generation circuit.
In the conventional induction heating apparatus, both of the oscillation period T and the duty ratio D of the high frequency generation circuit are changed in linear relation to the volume of adjustment S of the output setting knob as shown in FIGS. 1 and 2, and when the oscillation period T and the duty ratio D are simultaneously changed with respect to the volume of adjustment S of the output setting knob, the final heating output value P is proportional to the square of the volume of adjustment S of the output setting knob as shown in FIG. 3.
Therefore, when the heating output is set at a high level, change .DELTA.P in the heating output value is increased with respect to change .DELTA.S in the volume of adjustment of the output setting knob, leading to difficulty in setting of the heating output value.